In production lines such as, for example, assembly lines for vehicles etc., metal parts are connected by welding with the aid of a welding tool of a welding device. A welding gun having electrode caps is employed as a welding tool in the automatic assembly and welding of bodies. The electrode caps are contaminated on account of the welding operation, since coatings of the metal panels to be welded are deposited on the caps. In addition, contamination arises on account of the thermal process during welding. The electrode caps thus have to be cleaned at certain time intervals.
In a conventional manner, cleaning of the electrode caps is performed by means of a milling cutter which cuts off or mills away the contaminated part of the electrode cap (material: copper-chromium-zirconium; CuCrZr). The time interval between the milling operations is established in a fixed manner, having a certain maximum number of weld spots. The actual value of wear is determined by a counter (wear counter). This wear counter is incremented with each weld spot. The counter for the actual value is reset to 0 after each milling operation. Moreover, the time interval should be established so as to be dependent on the coating and the total thickness of the metal panels of the workpiece to be welded. For example, in the case of materials having a total thickness of the metal panels of less than 1.8 mm, milling is thus carried out after about every 200 weld spots; in the case of a thickness of 3 mm, the nominal preset value is approx. 400 spots. Milling has to be more frequent in the case of a hot-dip galvanized coating or an Al—Si coating than in the case of an organic coating.
It is problematic herein that a plurality of vehicle models having different model variants are produced on one assembly line and using one welding controller. Depending on how the vehicle models come onto this line, the most varied combinations of metal panels are welded. The sequence of the vehicles is random and changes continually. Thus the sequence of the combinations of metal panels which are processed within the defined milling interval also changes. Parallel thereto, the variations in the combinations of metal panels which are to be welded are steadily increasing. There are steadily increasing differences in the coatings, the latter increasingly varying in a significant manner. At the same time, the differences in the total thickness of the metal panels are also greatly increasing, on account of which the welding times vary very greatly, which likewise influences the wear of the electrode cap of the welding gun. As a result, it is impossible to establish a defined length of a milling interval. For this reason, the degree of contamination of the electrode caps within a fixed milling interval of consistent length may greatly vary today.
However, since the degree of contamination of the electrode cap influences welding quality, the degree of contamination of the electrode cap should be as uniform as possible. In the case of difficult combinations of metal panels, such as the connections of thin panels (two metal panels) or a thick-thick-thin connection (three metal panels), this is so critical that the mentioned variance of the degree of contamination of the electrode cap has such a serious influence that weld faults, for example open spots, may occur. Here, the quality of the operator's production line is impossible to evaluate for the operator of the welding gun in a production line.
However, the wear counter does not only control electrode care. The former likewise assumes the function of readjusting the welding current. That is to say that the user may determine within the milling interval by what percentage his welding current is to be increased at a certain state of wear of the electrodes. In the case of non-adaptive welding systems, this compensates for the influences of electrode contamination on welding quality.
The requirement for a consistent degree of contamination and optimum readjustment may be served, for example, in that the user may input a wear factor for each weld spot at the user interface of the welding controller. By way of this factor, the influence of the welding time of a spot may be adapted to electrode wear.
However, this procedure has two disadvantages:
On the one hand, the effort by the customer is high and the knowledge is often not available in a conscious manner, so that conformation of the factor is forgotten when the process is changed. The resulting consequence is that in more than 95% of all cases the factor remains unchanged at 1.
On the other hand, in the case of adaptive welding controllers, the welding time is adapted by the controller for each weld spot depending on the process. However, any extension of welding time is thus not incorporated into the electrode wear.